The present invention is generally directed to processes for the preparation of submicron and nanometer sized particles. More specifically, the present invention relates to processes for the preparation of submicron sized particles and smaller by a particle size reduction method wherein micron sized particles or larger, and including submicron sized particles, and mixtures of the aforementioned particles can be ground to submicron and nanometer sized dimensions using conventional grinding or milling equipment, for example, a ball mill or attritor. The improved process is accomplished primarily by the use of a "matrix assisted" milling media comprising an ionic nanocomposite grinding aid or adjuvant comprised of nanocomposite particles comprised of, for example, nanoscopic magnetic particles and at least one ionic host resin.
Conventional ball milling of industrial powders or pigments typically leads to particulates which are on the order of about 1 to 2 micrometers in diameter, and generally not less than about 0.1 micrometers in size under optimal or extreme conditions. See for example, "Dispersion of Powders in Liquids", 3rd ed., G. D. Parfitt Ed., Applied Science Publishers, 1981, the disclosure of which is incorporated by reference herein in its entirety. Moreover, attempts to mill or grind particles to their primary particle size or to sizes smaller than about 100 nanometers usually fail because of particle aggregation or agglomeration into larger particles from Van der Waals' forces between like particles.
The prior art discloses methods of preparing submicron and smaller particles, for example, where nanometer sized particles must be prepared or grown de novo, that is, built up from individual atoms constituting the desired particle. This approach is disadvantaged in a number of respects, for example, the method requires that the reactants be maintained at low instantaneous concentrations to avoid large particle formation or agglomeration; and the product particles are typically at high dilution so that isolation and purification are costly and labor intensive.
The present invention, in embodiments, provides a superior small particle preparative process wherein a particle size reduction process permits large size particles, for example, of from about 0.1 micrometers to 10,000 micrometers or more, to be reduced in size to particles with below about 0.1 micrometers or 100 nanometers utilizing conventional ball mill or grinding equipment, for example, in embodiments, to volume average diameters of from about 3 to about 30 nanometers. The processes of the present invention also permits submicron sized particles to be further reduced in size, for example, of from about 200 to about 800 nanometers to less than about 100 nanometers.
The processes and products of the present invention are useful in many applications, for example, as a variety of specialty applications including liquid developer compositions for electrophotographic, magnetographic, and ink jet imaging processes. In embodiments of the present invention, the preparative processes are useful in formulating dry and liquid containing small sized particulate marking materials for use in printing applications, and for the preparation of small sized particulate colored and weakly colored ferrofluids for use in known applications.
Magnetic fluids selected for the present invention include those fluids which are known in the art as "ferrofluids". Magnetic ferrofluids, or ferrofluids, are ultra-stable colloidal suspensions of magnetic particles in a liquid carrier. These fluids behave as homogeneous Newtonian liquids and can react with an external magnetic field. The liquid carrier or base may be hydrocarbon, fluorocarbon, silicone oil, water, ester, or similar liquid. Magnetic fluids are commercially available in a range of liquid carriers and display a saturation magnetization as high as about 1,000 gauss. These fluids may be produced by several different methods. Magnetic fluids were first produced it is believed by the long term grinding of magnetite in a hydrocarbon, such as kerosene, containing an appropriate dispersing agent such as oleic acid, as disclosed, for example, in U.S. Pat. No. 3,215,572. A review of the properties and behavior of magnetic fluids may be found in an article by R. E. Rosenweig entitled "Magnetic Fluids" in International Science & Technology, July 1966, pp. 48-56, the disclosure of which is incorporated herein by reference in its entirety.